Public awareness functions are information which is made available to the public with the purpose of creating awareness as to certain conditions or situations. Such functions may include, by way of example only, display of current time, temperature, traffic conditions, as well as intercom and public address.
Public address (PA) systems are often found in buildings where they function to provide building-wide audio controlled from a central location. PA systems may simultaneously announce information to all the available locations or an announcer may choose to limit the announcement to a subset of available locations. These systems are typically characterized by a wiring backbone threaded through the building, connecting a control area to enunciators such as speakers strategically distributed throughout the building.
Because of similar electrical properties among PA systems, intercom systems and paging systems, the wiring backbone in the building may support all three systems. Furthermore, the same wiring system may support distribution of a number of different audio sources, such as music from a CD player, for example.
A plurality of audio amplifiers is located in a central location for connecting to the wiring backbone various audio sources such as microphones, music reproduction devices and intercom/paging units. Each room or area of a building, in turn, likely contains one or more speakers. Existing PA systems typically use 25VAC or 70 VAC balanced constant voltage distribution schemes in order to minimize the speaker wire size necessary to distribute the audio signal to remote rooms. In such systems, a transformer at the output of the amplifier isolates and de-couples the amplifier from other electronics connected to the wiring backbone. Likewise, the loads, such as speakers at the terminal ends of the wiring backbone, are coupled through transformer connections. The output transformer typically steps up the amplifier voltage, while the input transformer steps down the voltage at the speaker terminals. Such systems have been installed extensively in schools, hospitals, airports, train stations, correction facilities, and other buildings where distribution of audio information is necessary.
Typically, many facilities which provide public awareness functions, such as public address or audio paging, also need to display the current time in physically separated areas. To this end, master/slave clock systems are often used to ensure synchronization of individual slave clocks in each room. Master/slave clock systems are those in which a plurality of slave clocks are distributed throughout a given area, but are all controlled from a master clock or controller. Master and slave clocks may be either analog or digital. Typically, each slave clock has its own timing mechanism, but responds to the master for purposes of setting and synchronizing. Thus, for example, after a power failure it is not necessary to reset each individual slave clock, but by manipulating the master clock all of the slaves can be returned to the correct time of day. Master/slave clock systems are useful, if not necessary, in various applications such as schools, hospitals, or airports where a large number of clocks are distributed throughout the facility. In those applications, the slave clock has several advantages: there is no need to set each clock when time is being reset or after power failure, and the slave clocks can be corrected by the master clock to keep them synchronized with the master and thus all telling the same time.
Proper integration of a master/slave clock system requires providing power and correction, or synchronization, signals to each slave clock. In new construction, it's relatively easy to wire a building for a clock system. Installing a clock system into an existing building, however, is difficult and expensive. The wiring usually includes separate lines for power and control, threaded from a master clock to each slave clock. To avoid the expense and trouble of hard wiring, the slave clocks may alternatively be battery powered, while distributing synchronization signals via radio frequency (RF) signals from a master controller. Although wireless installations have the advantage of avoiding the expense and trouble of wiring an existing structure, battery powered slave clocks with RF-distributed synchronization require more maintenance than hard wired systems (e.g., constant changing of batteries) and provide less reliable synchronization because of non-uniform attenuation of the correcting RF signal, which depends on the characteristics of the building. For example, in buildings consisting of steel and thick masonry walls, the signal strength of an RF synchronization signal degrades very rapidly over short distances and the signal may be very weak in the more remote rooms, thus resulting in unreliable time settings.
Consequently, what's needed is a system that can be installed in a building without the expense and trouble of running wiring throughout the building and yet has the reliability of a hard-wired system.